SWswehner·Jan 24, 2015
For one-time-pad encryption, both sides have a copy of the same secret key.
The name says "one-time," because if a key is used to encrypt two different plaintexts, information will leak.
In this implementation, the results of two encryptions may be based on the same bytes from the key file; not for all bytes, but for some. This is because it chooses bytes from the file (function generate_random_offsets() at https://github.com/pannous/xipher/blob/master/encrypt.c), then writes both the address of those bytes and the XOR'd value to the result of the encryption.
There is no provision to avoid picking the same offsets/key-bytes from one encryption-run to the next, except that they are chosen at random. Because the format of the encrypted messages is very simple, it can be easily determined for two encrypted messages, which key-bytes they sha
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re. Remedies: store between runs, remove 00 from file,define chunks in the file?
It's not too difficult to avoid sharing the secrets between two encryption runs; it is not even necessary to chose the key-bytes from the file at random. One could just use the key-bytes in sequence, send the address of the first key-byte, and store the next available key-byte for the next run. When the shared key-file doesn't have enough random bytes, because it has been used for encrypting too many message, it can error out -- which it should, but this implementation will never do that.